There are lakes at the bottom of the ocean. These are places where the water contains far more salt than usual, making it extremely dense. It sinks, pools, and refuses to mix with the surrounding seawater, creating perception-defying lakes that, despite being hundreds of meters deep, have their own surfaces and shorelines. One such lake features in “The Deep”—the second episode of Blue Planet II, which aired Saturday on BBC America and other networks. Illuminated by submersible headlights, and accompanied by choral music, cutthroat eels wriggle from the “shores” of the lake and dive into its midst for reasons unknown. Some are so overwhelmed by the salt that they go into shock, their sinuous bodies twisting into convulsing knots.

Two weeks ago, I described Blue Planet II as the “greatest nature series of all time.” I stand by that, and I’m also crowning “The Deep” as the greatest of the series’ seven episodes. It follows in the tradition of the original Blue Planet from 2001, which also voyaged into the abyss for its second episode. The result was groundbreaking. To devote 50 minutes of television to exploring the deep ocean seems, at first, like lunacy. Its perpetual darkness does not exactly make for compelling cinematography. And its low temperatures and crushing pressures make it so inaccessible that it has barely been explored, much less filmed.

And yet, Blue Planet succeeded amply. It took us to fantastical worlds, from belching hydrothermal vents to a whale fall—the decaying carcass of a sunken whale. It avoided the usual menagerie of clownfish, penguins, and sharks in favor of oddities like the improbably dentured fangtooth, slimy hagfish, and Phronima, a parasite that inspired H. R. Giger’s chest-bursting alien. It even showed us species that were new to science, like the Dumbo octopus, so named for the earlike flaps that protrude from its head.

In some ways, the sequel series treads the same ground as the 2001 original, featuring similar locations (the original ended on a brine lake), similar creatures, and even the same structure, sinking deeper and deeper as the minutes tick by. But it also considerably ups the ante on its predecessor. It features a love story between two shrimp. This species of shrimp, as larvae, will waft into a Venus’s flower basket sponge, and grow up trapped within its crystalline walls. It shows how abundant life can be in belching underwater volcanoes and the cold depths of Antarctica. It reveals bubbles of methane violently erupting from the ocean floor in front of a submersible’s headlights, like rockets launching in front of an angry sun.

I talked to the producer Orla Doherty about rising—or perhaps, sinking—to the challenge. This interview has been edited and condensed for the sake of clarity.

Ed Yong: Did you feel a sense of responsibility to live up to the legacy of that original deep ocean episode?

Orla Doherty: That episode blew everyone’s brains, my own included. When they told me they’d like me to make the deep episode, I kind of gulped. The largest habitat on Earth. The most unknown habitat on Earth. The hardest to get into. Oh, thanks a bunch, everyone. I remember being in that original planning meeting and thinking: Wow, I am literally out of my depth. My worst fear was that we’d make a poor man’s imitation of the original episode.

We did an awful lot of research: I have about 250 deep-ocean scientists in my inbox on a regular basis. In my first few weeks, Alex Rogers at Oxford told me about the shrimp and the sponge. I thought: Oh my god, it’s a love story in the deep ocean. We cannot leave the Galapagos without finding the Venus’s [flower] basket sponge.

Yong: When you plan a show like this, do you head out with a checklist of things to film, or do you venture into specific places and cross your fingers?

Doherty: You don’t just take out your submarine and go out exploring. Our first shoot, we tried to film shoals of lanternfish, spawning in biblical numbers. But that’s in the open ocean, where they can escape in three dimensions—and that’s what they did. It was a very informative disaster. It made me think that we should strike a balance between going for these animals that move around, and going to very fixed points where we know animals congregate.

With the brine lakes, I’d written a whole complicated storyboard about how I was going to show the toxicity of this world. But on the very first dive, an eel took the plunge and demonstrated in 45 seconds flat just what that place was all about. I owe that eel a lot. We always speak to just about everyone who’s been to these worlds, and we pick their brains about what we’d see. Nobody mentioned the eel, or that the eel might go swimming in the pool. It was one of those amazing moments, which felt like the deep really wanted us to see something and show it to the world.

The same goes for the methane volcano. [The episode features shots of methane bubbles erupting from the ocean floor.] Nobody had seen that before. We went back the next day and it was like nothing had ever happened. It was a flat empty seabed. We got these windows in time and the windows closed.

Yong: You also show a whale fall—the community of creatures that scavenge on the carcasses of sunken whales. How did you find one?

Doherty: Deep-sea scientists have worked hard on whale falls in the Pacific, but no one has observed one in the Atlantic. It’s something we really wanted to do. Tragically, whales do die, when they’re struck by ships or entangled. Eventually, our scientists were alerted to the fact that a whale had been found dead at the surface. We attached weights to it so it would sink quickly without being attacked by sharks, a pinger so we could find it with a sub, and a little GoPro camera. The scientists wanted to know how long it would take for the word to get out that this mighty beast had landed on the seafloor. It took 26 minutes for the first sixgill shark to arrive.

Nobody knew what would come, but I was absolutely adamant that I would have sixgills in the film. They’re majestic, ancient animals. The way they move ... everything about them says: “I don’t live in your world and I don’t come from where you come from.” We didn’t know that our whale fall would attract seven of them, and we didn’t know about their Jekyll-and-Hyde nature. They’d go from these languid, placid animals to absolute demons, tearing this whale’s carcass apart.

Yong: It seems that you included several shots to convey a sense of scale and perspective in an otherwise dark, featureless habitat. For example, there’s a scene in which your submersible descends into the abyss, and there’s a long manta ray swimming in the background.

Doherty: I couldn’t believe it when I got that shot. It’s about scale. But it also makes you stop and think: That’s an animal from the top 20 meters of the ocean. We’re way into the twilight zone, but we’re still seeing it. When you spend time in the deep, you realize that the animals we know don’t think: “Should I stop at 200 meters?” They keep going, and they use the deep in a way we’re only starting to comprehend. We showed swordfish hunting in the deep. I saw a mako shark at 700 meters down, which is not its deepest recorded depth. It showed me how interconnected the deep is to the rest of the ocean. It’s not a separate world. And showing the manta ray was a way of visually getting across that message.

Yong: I also noticed that many of the shots in the new episode seem far more atmospheric than the original. Is that because you got to use two submersibles?

Doherty: That is one of our key differences. If you’re just in a single sub, your camera is always pointing from the same direction from which your lights are shining. It’s really, really difficult to get shade, relief, shadows, and a sense of the landscape. But we shot half of this film from a research vessel with two submersibles. Through the extreme skill of the pilots, we could bring the extravagant lighting techniques that you’d use in a Hollywood movie to a depth of 1,000 meters. That just transformed how these worlds look.

Yong: In the making-of segment that follows the episode, there’s a scene in which you and your colleagues realize that your sub has sprung a leak. What was it like being in these tiny metal spheres, so far away from the surface?

Doherty: I spent 500 hours in the subs. It was really like going in a capsule and going off into orbit. Once you’re released from the ship, you’re on your own. Whatever goes on, you have to deal with it. The scrubber, which sucks the carbon dioxide out of the atmosphere, can begin to fail because it’s too cold or humid. The battery might burn faster because the water’s so cold. And when you’re filming on land and your camera stops working, you can tap something or change the battery. In the sub, the camera is outside and if it packs up, your dive is done. It’s a reminder that this is a world that humans really shouldn’t be in.

Yong: And yet, we kind of are. You show how thoroughly ships can destroy deep-sea reefs when they drag their nets along the seafloor. Past BBC series have tended to push environmental messages to a coda. What made you decide to interweave those messages throughout the entire series?

Doherty: We set out to make a series about the ocean. When I went out to film these incredible deep-sea coral beds, that’s what I found. While we were filming the Great Barrier Reef, it went through the greatest bleaching event it’s ever seen. These stories became embedded within these habitats, because that’s what we saw. We could have not included them, but that would have been disingenuous to what our ocean looks like today.

We want to hear what you think about this article. Submit a letter to the editor or write to letters@theatlantic.com.